Inductors are frequently formed in integrated circuits; however, given the inherent limitations of integrated circuit technology, it is difficult to form a high value inductor.
An inductor is generally created by forming a conductive coil around a core. The core may be an insulator or a magnetic core. Magnetic cores result in greater inductance values but are impractical to form in many types of integrated circuit. The inductance value is also greatly affected by the number of turns of the coil, where the inductance value is proportional to the square of the number of turns of the coil. Inductance value is also affected to a lesser extent by the radius of the coil and other well known factors.
Various methods have been used in an attempt to obtain high inductance values. Two such methods are described in U.S. Pat. No. 5,227,659 by Hubbard, and U.S. Pat. No. 5,095,357 by Andoh, et al, both patents incorporated herein by reference. In the '357 patent, it is disclosed that a high value inductor may be formed by two substantially flat spirals of metal, either arranged side-by-side or separated by an insulating layer, where an end of one flat spiral is connected to an end of the other flat spiral using an interconnection layer. Such a technique has certain drawbacks. One of the drawbacks is that the substantial length of the flat spirals may result in some destructive interference, due to phase opposition, in high frequency signals through the spiral. Another drawback is that the interconnection layer requires the formation of additional insulating layers and metal layers yet adds little or nothing to the inductance value.
In the '659 patent by Hubbard, a single, multi-level coil is described, where one coil turn is provided at each level. Hence, the inductors described in the '659 patent are limited to relatively few coil turns.
For most applications of an inductor, such as in a resonant circuit (also known as a tank circuit), the Q, or quality, factor of the inductor is important. The Q factor is the ratio of the reactance (X) of the inductor at a given frequency (f) to its DC resistance. The reactance of an inductor of value L is equal to 2 .pi.fL.
Accordingly, it is desirable to improve upon the existing inductors formed using integrated circuit techniques to obtain a high value inductor with a high Q factor.